1. Field of the Invention
This invention relates generally to semiconductor processing, and more particularly to apparatus and methods for attaching lids to integrated circuit substrates.
2. Description of the Related Art
Many current integrated circuits are formed as multiple die on a common wafer. After the basic process steps to form the circuits on the die are complete, the individual die are cut from the wafer. The cut die are then usually mounted to structures, such as circuit boards, or packaged in some form of enclosure.
One frequently-used package consists of a substrate upon which a die is mounted. The upper surface of the substrate includes electrical interconnects. The die is manufactured with a plurality of bond pads. A collection of solder bumps are provided between the bond pads of the die and substrate interconnects to establish ohmic contact. After the die is mounted to the substrate, a lid is attached to the substrate to cover the die. Some conventional integrated circuits, such as microprocessors, generate sizeable quantities of heat that must be ferried away to avoid device shutdown or damage. The lid serves as both a protective cover and a heat transfer pathway.
To provide a heat transfer pathway from the integrated circuit to the lid, a thermal interface material is placed on the upper surface of the integrated circuit. In an ideal situation, the thermal interface material ideally fully contacts both the upper surface of the integrated circuit and the portion of the lower surface of the lid that overlies the integrated circuit. Conventional thermal interface materials include various types of pastes, and in some cases, a metal. More recently, designers have begun to turn to indium as a thermal interface material.
The attachment of a lid to a die substrate involves a complex choreography of steps. The thermal interface material must be applied to the die. An adhesive must be applied to the substrate and cured in such a way that does not produce unwanted irregularities in the thickness or wetting of the thermal interface material. The lid must be attached to the substrate so that the tilt of the lid relative to the die is within acceptable tolerances. High tilt can lead to nonuniformities in thermal interface material thickness, which can produce poor heat transfer characteristics. A major factor in achieving acceptable lid tilt is the use of a proper fixture to hold the die substrates during lid processing.
One conventional fixture suitable for use with a pin grid array (PGA) die substrate uses a flat bottom plate upon which the PGA substrates are seated. After the lids are placed on the substrates, a middle plate is pressed down on the lids and the substrates are subjected to various thermal cycles to cure the adhesive. Depending on the composition and stiffness of the bottom plate, the thermal cycles can produce a warping of the bottom plate. Warping of the bottom plate can lead to misalignment of the lids and attendant tilt errors. This conventional fixture is not suitable for a class of die substrates known as land grid array (LGA) substrates. LGA substrates dispense with the downwardly projecting pins used in PGA substrates.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.